The Carbonate Silicate Geochemical Cycle at Ga for an Abiotic Earth Surface Just at the Transition to Biotic Colonization

I assume here that the late bombardment, with its potential for continual large sterilizing surface impacts, was over by 3.8 Ga, thus taking this time as the transition time to biotic colonization of the early continents (Oberbeck and Mancinelli 1994).

With the assumptions made above, the product of outgassing rate and land area ratios, (V/V)(AJ A), at t = 3.8 Ga for two limiting models (models a and c) and a preferred model (b) is given in table 8-3. The variation of A/Ao, V/Vo, and (V/VJ(Ao/A) for the above models is shown in figures 8-11, 8-12, and 8-13, respectively. The upper limit of the outgassing ratio, V/ V, is taken as 8 for the early Archean, approximately 30% higher than an estimate for 3 billion years ago made from a consideration of heat generation rates and depth of eutectic melting and source volatile outgassing (Des Marais 1985). The lower limit to land area is taken as 10% of the present value

figure 8-13.

(V/V) x (A J A) versus time for models a, b, and c. Value for model c at 3.8 Ga is 80 (not shown).

figure 8-13.

(V/V) x (A J A) versus time for models a, b, and c. Value for model c at 3.8 Ga is 80 (not shown).

(Allegre andJaupart 1985 gave values of 16% to 32% of the present continental mass at this time), and an upper limit to land area as 100% of the present value.

Isotopic dating reveals continental crust in existence at 4 to 4.3 Ga (Bow-ring et al. 1989). Armstrong (1968, 1981, 1991) has argued for a constant volume continental crust in existence for at least 4 billion years, a model that apparently fits the isotopic data (Nd, Sr, Pb, etc.) as well as the net continental growth models. In Armstrong's model, the rate of continental growth equals its rate of destruction in subduction zones. This controversy was revived by Bowring and Housh (1995), who favor the theory of a substantial early continental crust. Conversely, Taylor and McLennan (1995) and Mc-Culloch and Bennett (1994) favor the notion of more gradual growth, with less than 10% of present volume at 3.8 Ga (60% by 2.5 Ga). Vervoort et al. (1996) have suggested that neodymium isotopes in the most ancient Archean gneisses—the basis of Bowring and Housh's (1995) argument—were probably reset by metamorphism, based on Hf isotopic pattern in zircons. From a consideration of the Nb/U ratio of Archean basalts compared with the primitive and present day mantle ratios and other evidence, new support has emerged for rapid continental growth early in Earth history, with most of the present mass in place by 2.7 Ga (Sylvester et al. 1997; Hofmann 1997; Sylvester 1998). However, taking into account the apparent variation of the U/Th ratio in the depleted mantle over geologic time, Collerson and Kamber (1999) favor a growth model similar to that ofMcCulloch and Bennett (1994). This interpretation is in variance with those favoring early continental crust, with little growth since 3.5 Ga, based on detailed geochemical studies of Archean crust (Calderwood 1998; Sylvester et al. 1998).

Note that if higher sea-floor spreading rates prevailed in the Archean, the sea level should have been higher, thus favoring a lower estimate for land area relative to continental mass than today (Drever et al. 1988).

Extreme limits for (V/Vo)(Ao/A) are taken for the assumed values at 3.8 Ga. Model a, with the constant A = V = 1, is a lower limit to (V/V)(Ao/ A). Although a constant or slowly decreasing outgassing rate to present may seem implausible, it is compatible with estimates of the total present exo-genic carbon inventory and carbon outgassing fluxes derived from measured C/3He in MORB glasses and 3He outgassing fluxes (Marty and Jambon 1987; Marty 1989). A (V/ V)3 8 Ga = 3 assumes a heat generation rate of four times the present rate, with komatiitic oceanic ridge formation dissipating

FIGURE 8-14.

3.8 Ga model results: biotic enhancement of weathering ratio, now to 3.8 Ga versus surface temperature (75, 85, and 110°C). Computed results shown for model b and c for three a values.

FIGURE 8-14.

3.8 Ga model results: biotic enhancement of weathering ratio, now to 3.8 Ga versus surface temperature (75, 85, and 110°C). Computed results shown for model b and c for three a values.

30% more heat than the basaltic ridge at the same spreading rate (Nisbet 1987). To sum up, input parameters for our 3.8 Ga model are the ratios of outgassing rates and land areas, (V/V)38 Ga and Ga, respectively, atmospheric pCO2 and hence temperature (from a modified version of Kast-ing and Ackerman's 1986 greenhouse function described in the section on climate modeling), with the ratio of the present biotic enhancement factor to that at 3 . 8 Ga (BR) as the output . Three temperatures corresponding to growth conditions for thermophiles/hyperthermophiles were chosen

Calculations for the 3.8 Ga model give conditions values of BR = 7 to 2250 (preferred model b for variation of outgassing and land area, 49 to 2250; model a results, which give even higher BR values, are not given here) for temperatures of 75 to 110°C for an abiotic Earth surface just at the transition to biotic colonization (figure 8-14). If the Earth's atmosphere at this time contained 10 to 20 bars of carbon dioxide, surface temperatures could have approached 100° C (Kasting 1989), near the approximate upper temperature limit of hyperthermophilic microbes.

The range of BR values computed from the 3.8 Ga model is consistent with limits on the present biotic enhancement of weathering (B), derived from experimental and field studies ofweathering; in other words, values on the order of 100 to 1000 if the assumed estimate of (V/V)(Ao/A)38Ga is not much higher than the model b value (even model c is probably consistent with B = 100 to 1000 because the computed BR values are likely lower limits for the reasons outlined in the section on the sensitivity of scenario H).

We previously outlined a scenario for the colonization of land by extreme thermophiles soon after the origin of life (Schwartzman and Volk 1989, 1990), leading to the accelerated removal of carbon dioxide via reaction with silicates and decrease in temperature. A clarification to this scenario, already discussed, is that the origin of life may have occurred as early as 4.2 Ga, but the first likely opportunity for the persistence of surface life was at about 3.8 Ga. Temperatures for growth of extant thermophiles/hyperthermophiles range from 74°C (upper limit for cyanobacteria) to 85°C (methanogens) to 105°C for Pyrodictum (upper limit 110°C) (Brock 1986). Chemical weathering intensities (fluxes/unit land area) relative to the present rate {(V/ Vo)(Ao/A)} for the 3.8 Ga model, assuming steady state, range from 1 to 80 (model b, 12). Are these unreasonably high? If the present global physical denudation rate is some six times the chemical rate (Holland 1978), the maximum chemical denudation rate possible at the present mean continental uplift rate is six times the present rate. Thus, at 3.8 Ga, minimum mean uplift rates of 0.17 to 13 (for model b, 2) times the present rate are needed for a steady-state carbon dioxide level in the atmosphere/ocean system. If the mean uplift rates in the early Archean were significantly higher than today's rate (E. Nisbet, personal communication), then higher chemical denudation rates would be possible. Alternatively, a steady state was not achieved until later in the Precambrian as volcanic emissions declined and land areas increased.

Another barrier to achieving steady state was the sheer amount of carbon dioxide in the atmosphere at 3.8 Ga (for 10 bars, most of the carbon dioxide would be in the atmosphere as a result of a low pH ocean; see Walker 1985). The removal time of a 10 bar carbon dioxide atmosphere is estimated to be on the order of 108 years computed by dividing an exogenic inventory corresponding to 10bars (2 X 1021 moles) by {(V/V)3.8Ga X Vo}, taking (V/ V)3 8 Ga = 3, Vo as the volcanic carbon flux (6 X 1012 moles/year; Berner et al. 1983) to the atmosphere/ocean. Because land area, volcanic outgassing rate, and solar luminosity change significantly over 108 years, a steady-state approximation is not entirely justified. However, the steady-state model is used for its computational simplicity because the focus is on the role of biotic enhancement of weathering. Thus, the model calculations represent at-tractors toward which the system is relaxing at any point in time, and not the actual values, which could lag behind by several hundred million years. A lag time of this magnitude would apply until the mid-Archean, when atmospheric carbon dioxide levels decreased dramatically, making the steady-state model a much better approximation to real behavior (e.g., see the Phanero-zoic model ofBerner 1990a).

Transient models have been computed by Tajika and Matsui (1990) and Walker (1991). Tajika and Matsui (1990, 1992, 1993) assume that initially all the Earth's carbon (aside from that in the core) was in the surface reservoir because of the partitioning of carbon between the proto-atmosphere and early magma ocean. Thus, in their model, the degassed carbon during Earth history comes from the release from subducted carbonate and a return flux from a regassed component derived from the subducted carbon lost to the mantle. However, because the speciation of carbon in the mantle is not well understood (SiC may be an important phase according to recent evidence; see Leung et al. 1990), it may be premature to conclude that the Earth's early mantle lacked levels of carbon high enough to supply a significant primordial flux through degassing over geologic time. Walker (1991) supports this viewpoint. Furthermore, the postulated regassing in the Archean is perhaps implausible in view of the apparent thermal barrier (Des Marais 1985; Abbott andLyle 1984).

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